Following attendance at the Marine Officers' School at Port Royal, South Carolina, he served as a member of the Marine detachments aboard Wisconsin and Delaware. In August 1912, he was assigned to Nicaragua, where he participated in the bombardment, assault and capture of the hills called Coyotepe and Barranca. Further foreign shore duty followed in the Philippines and China with the First Brigade and with the Marine detachment, American Legation, Peking, China, from 1913 to 1916.

In March 1916, Geiger joined Naval Air Station Pensacola, Florida, as a student naval aviator. He successfully completed the course and was designated a naval aviator in June 1917. He was designated Naval Aviator # 49 (Marine Corps Aviator # 5) on June 9, 1917.[4]

Further training followed and in July 1918, he arrived in France. He served with 5 Group, Royal Air Force at Dunkirk. He commanded a squadron of the First Marine Aviation Force and was attached to the Day Wing, Northern Bombing Group. He was detached to the United States in January 1919. For distinguished service in leading bombing raids against the enemy, he was awarded the Navy Cross.

In August 1927, he returned to Quantico as a squadron officer and instructor at the Marine Corps Schools, and in May 1928, was assigned to duty in the Aviation Section, Division of Operations and Training, at Marine Corps Headquarters. After attending the U.S. Army War College and graduating in June 1929, he was ordered to Quantico, where he was assigned duty as commanding officer, Aircraft Squadrons, East Coast Expeditionary Force. He returned to Washington and served as the officer in charge, Marine Corps Aviation from 1931 to 1935, a billet currently held by a lieutenant general that is now known as the deputy commandant for aviation.[5]

In June 1935, he returned to Quantico as commanding officer, Aircraft One, Fleet Marine Force. From June 1939 to March 1941, he was a student at the Senior and the Advanced Courses, Naval War College, Newport, Rhode Island. This was followed with a brief tour of duty in the Office of the Naval Attaché, London.

In April 1941, Geiger made his way from Lisbon to Gibraltar, where he changed from civilian clothes to his military uniform. He had lunch with the governor at Government House, in a visit which lifted British morale in Gibraltar. He was on his way to the Western Desert, as the first U.S. military observer attached to the British 8th Army. In August, he became commanding general, 1st Marine Aircraft Wing, Fleet Marine Force, in which capacity he was found upon the United States' entry into World War II.

On September 3, 1942, he was stationed at Guadalcanal to lead the Cactus Air Force during the early part of the Guadalcanal Campaign. Until November 4, he was commander of the combined Army, Navy and Marines Air Forces stationed here, of which the 1st Marine Aircraft Wing was part. He was awarded a Gold Star in lieu of a second Navy Cross for his service on Guadalcanal.

His citation reads in part, "Despite almost continuous bombardment by enemy aircraft, hostile naval gunfire and shore based artillery, the combined total of Army, Navy and Marine Corps units stationed at Guadalcanal under Major General Geiger's efficiently coordinated command succeeded in shooting down 268 Japanese planes in aerial combat and inflicting damage on a number estimated to be as great ... Sank six enemy vessels, including one heavy cruiser, possibly sank three destroyers and one heavy cruiser, and damaged 18 other ships, including one heavy cruiser and five light cruisers."

Geiger was recalled to Marine Corps Headquarters in May 1943, to become director of aviation. In November 1943, he returned to the field, this time as commanding general of the I Amphibious Corps and led the corps from November 9, to December 15, 1943, in the Battle of Bougainville, for which he was awarded the Distinguished Service Medal.

Redesignated III Amphibious Corps in April 1944, he led this organization in the invasion and subsequent recapture of Guam during July and August 1944, and in the assault and capture of the southern Palau Islands in September and October of the same year. For those operations he was awarded two Gold Stars in lieu of a second and third Distinguished Service Medal.

"General Roy S. Geiger Memorial Parkway" sign on County Road 220 in Clay County, Florida, just south of Jacksonville

He returned to Washington, D.C., and Pensacola, Florida, in September and October 1945 before resuming his duties.[8]General Geiger was transferred to Headquarters, U.S. Marine Corps in November 1946.[9][10]

Going ashore with the early landing elements on April 1, 1945, he began a bitter three-month campaign ... with outstanding professional skill, forceful leadership and unswerving determination, he directed his units ... repeatedly disregarding personal safety to secure a first hand estimate of the battle situation and inspiring his men to heights of bravery and accomplishment.

^Camp's article states that Geiger's transfer back to headquarters was "in late 1945." This is an obvious typographical error. Other sources such as Wellons, 2007, state actions taken by Geiger while in command of the Fleet Marine Force, Pacific during 1946 and make clear that his command ended in November 1946.